1. Field of the Invention
This invention relates to fiber optic communication, and more particularly, to high-speed drivers for interfacing with and enhancing the performance of vertical-cavity surface-emitting laser (VCSEL) diodes used within fiber optic communication systems, as well as in other applications.
2. Description of Related Art
Optical transmission systems have three general components: the light source, the transmission medium, and the detector. Light sources for an optical transmission system are typically either Light Emitting Diodes (LEDs) or lasers. (Semiconductor lasers have distinct advantages over LEDs, including higher data rates and longer distance transmission capabilities.) Typically, a pulse of light from the light source indicates a one bit and the absence of light indicates a zero bit. The transmission medium is commonly ultra-thin glass fiber. The detector generates an electrical pulse when light falls upon it.
Low-cost, high-performance, highly integrated fiber optic interface circuits are becoming increasingly necessary to meet the demands of high-speed digital data communication. With the advent of gigabit Ethernet systems, for example, fiber optic technology has become increasingly preferred. A fiber optic transmission line preferably uses a VCSEL diode as the light source to transmit optical data. In contrast to edge-emitting lasers, VCSELs have a vertical optical cavity that is perpendicular to the epitaxial growth direction. Beams emitting from an edge-emitting laser are highly astigmatic, making them less desirable in high-speed digital data communication applications. VCSELs typically emit a circularly symmetric Gaussian beam which is very conducive to high-efficiency coupling into optical fiber.
The high-speed nature of fiber optic communication necessitates that the VCSEL diodes operate quickly, accurately and efficiently. To enhance the operation of the VCSEL diodes, new driver circuits designed specifically for the VCSEL diodes are needed. These new driver circuits need to address the speed demands of fiber optic communication systems, including the need for higher edge rates and integrated edge enhancement circuitry, low-voltage differential signaling input interfaces (LVDS), low overhead current requirement, good power supply rejection, low power supply requirement, optimization for common-cathode VCSEL connection, and multiple parallel driver integration.
A typical prior art driver circuit for a laser diode appears in FIG. 1. Transistors T10 and T15 make up the differential input circuit, using input ports 10 and 15. Included also are two current mirrors, the first being formed from transistors T20 and T25. This current mirror sets the first reference current, IRefl, from current source 20. The second current mirror, formed by transistors T30 and T35, sets the second reference current, IRef2, from current source 25.
As a result of this prior art configuration, the dc bias current is required to flow even when the laser is off. This requires a high overhead current supply, which is undesirable. Because burst-mode optical transmitters require lasers to be off more than they are on, this constant current consumption is inefficient. The constant current draw also discourages driving multiple common cathode connected VCSELs or integrating multiple parallel drivers for use with VCSEL arrays. The relatively high current demands of such configurations do not integrate well with the present low-power fiber optic communications systems, which typically incorporate LVDS interfaces.
Another limitation of the prior art circuit in FIG. 1 is that it contains no integrated edge-rate enhancement circuitry and no precise current controls. In FIG. 1, transistor T15 switches the drive current to the laser diode. Gate-drain parasitic capacitance of transistor T15 typically results in voltage spiking that can drive the laser to emit light longer than it should, causing "overshoot" and thus undesirably limiting the efficiency and speed with which the VCSEL can operate.
Yet another limitation of the prior art circuit in FIG. 1 is that it has a narrow range of available driving current, which undesirably restricts its adaptability to drive VCSELs having different current requirements. This limitation further restricts using a plurality of common cathode connected VCSELs and further limits multiple parallel driver integration.
Still another limitation of previous laser driver circuits is common anode connection. Common anode driving of VCSELs can be undesirable because contact with a common anode-connected VCSEL can cause a harmful electrical discharge. Not only can this pose a safety risk, but also damage to the VCSEL and related circuitry may result.
There is a need in the industry for a driver circuit for a VCSEL diode that is cost-effective, safe, and preferably fabricated with complementary metal oxide semiconductor (CMOS) technology, while at the same time addresses the problems of typical driver circuits as outlined above.